I replaced my old AC only thermostat with Honeywell RTH6580WF smart Thermostat. Also replaced existing 3 terminal wire with 4 for C wire hookup.
TSTAT specs h t t p yourhome.honeywell.com/en/products/thermostat/wi-fi-7-day-programmable-thermostat-rth6580wf
Current TSTAT wiring.
RED - R (jumper R and Rc) YELLOW - Y (color has faded on it as you can see in image, I think its brown color, which is connected to white terminal from green pig tailed brown wire coming from outside fan unit) GREEN - G
Below is old picture of TSTAT wire (brown wire non pig tailed) connected to furnace wiring before I snaked 4 terminal wire.
On furnace side there are 2 brown wires, 1 has green wire pig tailed and other don't. The one which is pig tailed is coming from outside FAN unit while other brown wire is coming from TSTAT. Any idea where should I connect the blue wire from TSTAT to furnace side C Wire? (you don't see in attached picture as its the old TSTAT wire with 3 terminal)
I am thinking it should be connected to Brown and Red wire (FIG-2) using wire nut. I do get 28v between original Red and Brown wire making me think that brown is C wire. Is that right?